1. Field of the Invention
The present invention relates generally to electrical wiring devices, and particularly to electrical wiring devices having protective features.
2. Technical Background
An electric distribution system transmits AC power from a breaker box to one or more load circuits disposed in a structure to provide electrical power throughout. A load circuit may include any number of electrical devices such as electrical outlets, lighting devices, appliances, or other such devices. An electric circuit typically includes at least one protection device. Examples of electric circuit protection devices include ground fault circuit interrupters (GFCIs), arc fault circuit interrupters (AFCIs), or devices that include both GFCIs and AFCIs in one protective device.
A protective device is mounted in an upstream outlet box within the electric circuit and non-protective devices, such as receptacles, are mounted downstream of the protective device within the electric circuit. Electrical wiring is placed within the structure between the breaker box and the various outlet boxes in the circuit. At the protective device location, a portion of the electrical wiring is fed into the outlet box. The portion of the electrical wiring is cut into two pieces. For example, the upstream portion of the electrical wiring (i.e., the line cable) is connected to the line terminals of the protective device such that the protective device is connected to the AC power source. The downstream portion of the cable (i.e., the load side cable) is connected to the load terminals of the protective device. The remainder of the load side cable extends to the remainder of the electrical devices (e.g., electrical receptacles) in the electric circuit. A connection process is performed at each outlet box until the terminals of the last device are connected to the electrical wiring.
A protective device typically includes one or more integral face receptacles accessible to a user via the front face of the device. Thus, an electrical appliance with a corded plug may be plugged into the receptacle to obtain power. The electrical loads that may be serviced by the protective device include loads connected to the face receptacles, the downstream wiring, downstream receptacles, user attachable loads plugged into the downstream receptacles, and permanently connected loads (e.g., lighting). When everything in the electric circuit is operating properly, the protective device provides power from the AC power source to the loads in the electric circuit.
As its name suggests, a protective device protects the load circuit from one or more fault conditions. One type of fault condition is known as a ground fault condition. A ground fault may occur, for example, by frayed or missing insulation on a hot conductor disposed somewhere in the load circuit. If a human being (or some other conductive element) were to simultaneously contact the hot conductor and a ground path a current would flow to ground through the person. This current is potentially lethal. Fortunately, the protective device (GFCI) is configured to detect and interrupt the resulting current flow through the body before there is serious injury or even electrocution. Another type of fault condition is a parallel arc fault. This type of fault occurs when there is damaged insulation between a hot conductor and an adjacent conductor (that is at a different potential). The damaged insulation allows a sputtering current to flow across the compromised insulation. A series arc fault represents another type of fault condition. A series arc fault occurs because a termination in the load circuit is loose. For example, a wire nominally terminated by the screw terminal of an electrical device (e.g., an outlet receptacle or a switch) may be loose because the screw terminal is not completely tightened; a small gap may be formed between the wire and the screw terminal. As another example, when a wire is accidentally severed, a small gap may be formed at the cut such that adjacent ends of wire are almost touching. In each instance, a sputtering arc fault may bridge the small gap. The fault current is limited by the impedance of the load. Series arc fault conditions can also occur in the line cable or elsewhere upstream of the AFCI. The protective device (AFCI) senses and detects at least one of these types of arcing conditions and interrupts the current flowing through the fault before there an electrical fire is started. There are other types of protective devices other than the ones described above, such TVSS devices, GFEP devices, etc. The aforementioned protective devices are non-limiting examples of such devices.
One drawback to all prior art electrical devices is that they are subject to one or more end of life conditions. An end-of-life condition refers to a failure that should render the device unusable or unsafe for use. For example, some end of life conditions may make a protective device non-protective. This drawback may be addressed by providing an end-of-life monitoring circuit that is configured to detect the end of life condition and interrupt any unprotected power to the load circuit. A device of this type may also include an end-of-life display that provides a signal to the user indicative of the end-of-life state. Upon learning of the condition, the user would be required to replace the device to resume service to the load circuit. An end-of-life indicator of this type may provide either a visual or audible indication that warns the user that the protective device needs to be replaced.
Another drawback to prior art protective devices relates to the fact that can be miswired during installation. Since the protective device has line terminals and load terminals it is possible to make the mistake of connecting the line cable to the load terminals and the load cable to the line terminals; this condition is commonly referred to a miswiring or reverse wiring. When reverse wired, some prior art GFCIs are not capable of protecting the face receptacles. One approach for solving the problem has been to provide product labeling and installation instructions sheets that warn against miswiring. These have lessened the chances for miswiring but unfortunately some installers choose to ignore installation instruction sheets. Another approach to the aforementioned problem is to include a miswire detection circuit configured to detect a miswired condition and automatically prevent the protective device from resetting. As a result, no power is provided to the downstream circuit or the face terminals. The lack of power eventually induces the installer to correct the miswired condition. While this approach may be successful for an initial GFCI installation, it may not be operative for subsequent reinstallations. Protective devices that do include miswire detection for subsequent installations often include relatively expensive solutions to the problem.
What is needed, therefore, is a protective system that inexpensively detects end of life conditions and miswire conditions in first and subsequent installations.